1. Field of the Invention
This invention relates to a photoelectric tube and more particularly to a photomultiplier tube having an improved alkali metal vapor generator for directing the alkali metal vapor substantially toward the photocathode surface.
2. Description of the Prior Art
Many photoelectric tubes such as image intensifier tubes, camera tubes and photomultiplier tubes have photoemissive cathodes or secondary electron emissive surfaces obtained by means of an alkali metal vapor source within the tube envelope. The quantum efficiency and spectral response of these surfaces have been improved by the deposition of multi-alkali metals, such as sodium, potassium and cesium on a substrate of antimony. The known vapor sources utilized to obtain such surfaces typically comprise a metal holder or channel which contains a chromate of the desired alkali metal and a reducing agent for the chromate as described, for example, in U.S. Pat. No. 3,372,967 to Hughes, issued on Mar. 12, 1968. Such channels may be heated to vaporize the alkali metal by well-known direct current or induction heating methods.
In a photomultiplier tube, for example, one or more of the channels are typically mounted near the dynode cage section of the tube, as shown in U.S. Pat. No. 3,658,400 issued to Helvy on Apr. 25, 1972. One problem caused by this construction is that the evaporation of the alkali metal is released uncontrollably from the channels depositing alkali metal vapor on the internal parts as well as on the faceplate surface. Physical restrictions of the vapor deposition due to the geometry of the tube internal components often result in lack of uniformity in cathode thickness, in particular with multiple evaporations. The capability of reproducing cathodes of consistent sensitivities is diminished with the result being undesired spectral variations from tube to tube. This deposition control is further complicated where antimony dynodes are required. The combination of tube geometry and antimony dynodes, having an affinity for the alkali metals, further denies the photocathode surface of receiving an adequate deposit of alkali metal for proper activation.
Another problem concerns the use of the alkali metal vapor sources in tubes to be operated in severe vibrational conditions. These vapor sources must remain in the tube after the evaporating processes, performed after or during tube evacuation, are complete. Severe shock and vibration often shake loose particles of alkali metal from the channels of the known vapor sources. These excess particles can cause undesirable short circuits between components at different potentials. Open circuits can also be caused by erosion of metal contacts due to the abrasiveness of the particles. The overall reliability of the tube under such conditions is thereby reduced.